Apparatus and method for controlling optical gain profiles

ABSTRACT

The invention includes an apparatus for processing an input optical beam. The apparatus has at least one variable optical element to dynamically alter the polarization state of an optical beam to form a polarization-altered optical beam, wherein the polarization-altered optical beam includes elliptical polarization. At least one wave plate operates on the polarization-altered optical beam, each wave plate has a selected retardation, order of retardation, and orientation. A polarization analyzer, operating in conjunction with the at least one variable optical element and wave plate, alters the transmitted amplitude of the polarization-altered optical beam as a function of wavelength, and thereby produces an output optical beam with transmitted amplitude adjusted as a function of wavelength.

This application claims priority to U.S. provisional patent applicationNo. 60/306,663, filed Jul. 19, 2001, entitled “A Dynamic Gain TiltControl Device now abandoned.”

BRIEF DESCRIPTION OF THE INVENTION

The present invention relates generally to optics, fiber optics, andoptical networks. More particularly, the present invention relates tothe control of optical gain profiles through the use of a dynamic gaintilt and curvature control device that has applications in opticalnetworks, optical communications, and optical instrumentation.

BACKGROUND OF THE INVENTION

Optical fibers are replacing copper cables at a rapid pace as thetransition medium for communication systems. Optical fibers are used inthe long-haul telecommunication backbone, as well as in regional andmetropolitan systems to service the fast growing need of wider bandwidthand faster speed fueled by Internet usage. A dramatic increase in theinformation capacity of an optical fiber can be achieved by thesimultaneous transmission of optical signals over the same fiber frommany different light sources having properly spaced peak emissionwavelengths. By operating each source at a different peak wavelength,the integrity of the independent messages from each source is maintainedfor subsequent conversion to electric signals at the receiving end. Thisis the basis of wavelength division multiplexing (WDM). To ensure smoothand efficient flow of information, optical networks should haveintelligence built in. Dynamically controllable devices are one of thekey building blocks for smart optical networks.

For optical signals to travel long spans in optical networks based onWDM or dense WDM (DWDM) without expensiveoptical-to-electrical-to-optical (OEO) conversion, optical amplifiersare used. Today, the most often used optical amplifiers are Erbium DopedFiber Amplifiers (EDFAs). Optical networks need to have uniform powerlevels across the channels to minimize detection noise and signalsaturation problems. However, in practice, the widely used EDFAs havenon-linear gain profiles. For static EDFA gain, the gain profile can becompensated by passive gain flattening filters (GFF) based on thin filmdielectric filtering or Bragg grating technologies. However, in additionto this static problem, there are several factors that cause dynamicwavelength dependent gains in the optical networks. These factorsinclude (a) saturation effect of the amplifier medium; (b) pump laserpower and different gain settings; and (c) number of channels (changesdue to adding and dropping channels) input powers.

FIG. 1a shows the measured gain profiles of an EDFA (with a GFF) set atdifferent gains. It is evident that the gain profile tilts significantly(>10 dB) over the 1528 nm to 1563 nm wavelength band. In addition, thereis a noticeable change in slope below approximately 1540 nm when thegain setting is changed from 20 to 10 dB. The GFF in this case has beenoptimized for 23 dB amplification. As a rule, if the gain is set belowthe GFF-optimized gain, the tilt is positive; if the gain is set abovethe GFF-optimized gain, the tilt is negative. This is illustrated inFIG. 1b, which shows the tilt for an EDFA that has been gain-flattenedwith a GFF at 20 dB.

To compensate dynamically for this wavelength dependent gain, dynamicgain equalizing (DGE) devices have to be used. Several DGE solutionshave been proposed. They fall into two general categories: dynamicchannel equalizer (DCE) and dynamic spectral equalizer (DSE). Dynamicchannel equalizers use a grating or thin film filters to de-multiplex(demux), control, and then multiplex (mux) individual channels toachieve equalization. Although they offer good flexibility down toindividual channel levels, they are sensitive to channel number andspacing, and thus not scalable. They are also complicated in design,large in package size, and very expensive (>$1000/channel).

Dynamic spectral equalizers, on the other hand, only control the overallspectral shape without the demux/mux steps and offer importantscalability. They can be used for different DWDM systems with differentchannel numbers and spacing. The current solutions are based on multiplestage systems, where each stage controls a portion of the spectrum.Their disadvantages include high cost, high insertion loss, andreliability. They are still complicated in design, packaging (e.g.alignment of many stages), and control. Therefore, they are stillexpensive (>$5000).

A known optical control technique is tunable optical retardation.Tunable optical retardation can be implemented in a number of ways. Forexample, a liquid crystal can be used to implement this function. Liquidcrystals are fluids that derive their anisotropic physical propertiesfrom the long-range orientational order of their constituent molecules.Liquid crystals exhibit birefringence and the optic axis can bereoriented by an electric field. This switchable birefringence is themechanism underlying nearly all applications of liquid crystals tooptical devices.

A liquid crystal variable wave plate is illustrated in FIG. 2a. A layerof nematic liquid crystal 1 is sandwiched between two transparentsubstrates 2 and 3. Transparent conducting electrodes 4 and 5 are coatedon the inside surfaces of the substrates. The electrodes are connectedto a voltage source 6 through an electrical switch 7. Directly adjacentto the liquid crystal surfaces are two alignment layers 8 and 9 (e.g.,rubbed polyimide) that provide the surface anchoring required to orientthe liquid crystal. The alignment is such that the optic axis of theliquid crystal is substantially the same through the liquid crystal andlies in the plane of the liquid crystal layer when the switch 7 is open.

FIG. 2b depicts schematically the liquid crystal configuration in thiscase. The optic axis in the liquid crystal 1 is substantially the sameeverywhere throughout the liquid crystal layer. FIG. 2c shows thevariation in optic axis orientation 12 that occurs because of molecularreorientation when the switch 7 is closed.

As an example, we consider a switchable half wave retardation plate. Forthis case, the liquid crystal layer thickness, d, and birefringence, An,are chosen so that $\begin{matrix}{\frac{\Delta \quad {nd}}{\lambda} = \frac{1}{2}} & (1)\end{matrix}$

where λ is the wavelength of the incident light. In this situation, iflinearly polarized light with wave vector 13 is incident normal to theliquid crystal layer with its polarization 14 making an angle 15 of 45degrees with the plane of the optic axis of the liquid crystal, thelinearly polarized light will exit the liquid crystal with itspolarization direction 18 rotated by 90 degrees from the incidentpolarization.

Referring now to FIG. 2c, the optic axis in the liquid crystal isreoriented by a sufficiently high field. If the local optic axis in theliquid crystal makes an angle θ with the wave vector k of the light, theeffective birefringence at that point is $\begin{matrix}{{\Delta \quad n_{eff}} = {\frac{n_{e}n_{o}}{\sqrt{{n_{o}^{2}\cos^{2}\Theta} + {n_{e}^{2}\sin^{2}\Theta}}} - n_{o}}} & (2)\end{matrix}$

where n_(o) and n_(e) are the ordinary and extraordinary indices of theliquid crystal, respectively. The optic axis in the central region ofthe liquid crystal layer is nearly along the propagation direction 13.In this case, according to Eq. 2, both the extraordinary 16 and ordinarycomponents 17 of the polarization see nearly the same index ofrefraction. Ideally, if everywhere in the liquid crystal layer the opticaxis were parallel to the direction of propagation, the medium wouldappear isotropic and the polarization of the exiting light would be thesame as the incident light.

Before leaving a discussion of this tunable half wave plate, it isuseful for later understanding of the current invention to give ageometrical representation of the polarization as afforded by thePoincare sphere. FIG. 3 shows a projection of the Poincare sphere asviewed from the top. In this view, circular polarization 21 is at thecenter of the projection; all states of linear polarization occur on theequator—the outer most circle. Two diametrically opposed points on thesphere correspond to orthogonal polarizations. For example, the twopoints 22 and 23 represent orthogonal linear polarizations, as do points24 and 25. When light propagates through a liquid crystal layer, or anyother birefringent medium, its polarization will change continuously;this change can be mapped as a continuous curve on the sphere. The curve26 shown on the sphere in FIG. 3 represents the changes in polarizationthat are experienced for the situation of FIG. 2b. Point 22 correspondsto the incident polarization and point 23 to the exit polarization ofthe unactivated liquid crystal cell. Observe that they are orthogonal.

In view of the foregoing, it would be highly desirable to provide asingle-stage solution for dynamic (or tunable) gain tilt compensation.Ideally, such a solution would utilize tunable optical retardation,simplified control techniques, and could be implemented in a verycompact package. In addition, such a solution would ideally provide amechanism for closely fitting a non-linear spectral profile.

SUMMARY OF THE INVENTION

The invention includes an apparatus for processing an optical beam. Theapparatus has at least one variable optical element to dynamically alterthe polarization state of an optical beam to form a polarization-alteredoptical beam, wherein the polarization-altered optical beam includeselliptical polarization. At least one wave plate operates on thepolarization-altered optical beam, each wave plate has a selectedretardation, order of retardation, and orientation. A polarizationanalyzer, operating in conjunction with the at least one variableoptical element and wave plate, alters the transmitted amplitude of thepolarization-altered optical beam as a function of wavelength, andthereby produces an output optical beam with the transmitted amplitudeadjusted as a function of wavelength.

The invention also includes a method of processing an optical beam. Themethod includes dynamically altering the polarization state of anoptical beam to form a polarization-altered optical beam, wherein thepolarization-altered optical beam includes elliptical polarization. Thetransmitted amplitude of the polarization-altered optical beam isaltered as a function of wavelength, thereby producing an output opticalbeam with the transmitted amplitude adjusted as a function ofwavelength.

BRIEF DESCRIPTION OF THE FIGURES

The invention is more fully appreciated in connection with the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIGS. 1a and 1 b show the gain tilt of an Erbium-doped fiber amplifier(with gain flattening filter) for different amplifier gain settings. InFIG. 1a the amplifier is combined with a gain-flattening filter that isoptimized for 23 dB of gain; FIG. 1b has a static gain-flattening filteroptimized for 20 dB.

FIGS. 2a-2 c illustrate the basic operation of a liquid-crystal-basedelectrically adjustable wave plate.

FIG. 3 shows a Poincare sphere representation of the polarizationchanges induced by a half wave plate.

FIG. 4a illustrates a dynamic gain tilt compensation deviceincorporating one fixed wave plate; FIG. 4b illustrates a wave plateorientation of 45 degrees.

FIGS. 5a-5 c show the performance of the embodiment of FIG. 4a. FIG. 5aillustrates how the variable wave plate retardation effects the changesin polarization from the minimum to maximum wavelength using thePoincare sphere representation. FIGS. 5b and 5 c show the attenuationversus wavelength when the fixed wave plate is optimized for negativeand positive slope, respectively.

FIG. 6a illustrates a dynamic gain tilt compensation device employingtwo fixed wave plates; FIG. 6b illustrates wave plate orientations.

FIGS. 7a-7 c show the simulated performance of the embodiment of FIG.6a. FIG. 7a shows the attenuation versus wavelength when the variablewave plate retardation is between 1.0 and 1.3 waves. FIG. 7b illustrateshow the variable wave plate retardation effects the changes inpolarization from the minimum to maximum wavelength using the Poincaresphere representation. FIG. 7c shows the attenuation versus wavelengthwhen the variable wave plate retardation is between 0.7 and 1.0 waves.

FIG. 8 illustrates measured results for the embodiment of FIG. 6a.

FIG. 9 illustrates simulated results for a design with three wave platesand one variable wave plate.

FIG. 10A is a schematic of an embodiment of the current invention foroperation in a fiber optic line; FIG. 10B illustrates wave plateorientations.

FIG. 11 is a schematic of an embodiment of the current invention forcontrolling the gain tilt of a fiber amplifier.

Like reference numerals refer to corresponding parts throughout theseveral views of the drawings.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a method and device for dynamicallyadjusting the wavelength profile of the intensity of an optical signal.The method incorporates an adjustable wave plate in combination with oneor more high order fixed wave plates to tailor the polarization of thelight versus wavelength. In combination with other optical elements suchas polarizers and beam splitters, the polarization variation can beconverted into an intensity variation with wavelength of the outputoptical beam. Such a device is particularly suited to controlling theoutput profile of a fiber amplifier (e.g. EDFA) to compensate for thewavelength dependence of the amplification for WDM applications. Theadjustable wave plate can be of any type (e.g. birefringent crystalwedge, electro-optic, acousto-optic, or liquid crystal) and control maybe mechanical, electromechanical or electronic depending on the type ofwave plate that is used.

The invention provides dynamic control of optical gain profiles. Certainembodiments are directed toward compensation for gain tilt and curvatureas described in the background section. For illustrative purposes, theadjustable wave plate will be chosen as an electrically addressed liquidcrystal cell, shown schematically in FIG. 2a. The basic concept of theinvention is illustrated in FIG. 4a.

In FIG. 4a, a beam of linearly polarized light 27 is applied to avariable optical element in the form of a liquid crystal cell 28. Thelight passes normally through the homogeneously aligned liquid crystalcell 28 whose optic axis 29 makes an angle 30 of 45 degrees with thedirection of polarization, as shown in FIG. 4b. After passing throughthe liquid crystal, the light passes normally through a static waveplate 31 whose optic axis is parallel to the optic axis of the liquidcrystal. The wave plate 31 has a selected retardation, order ofretardation, and orientation. After passing through the static waveplate 31, the light is in general elliptically polarized. As usedherein, elliptical polarization can include circular polarization. Thedegree of ellipticity and the orientation of the major axis of theellipse depend on the wavelength of the light. A polarization analyzer32 is positioned after the wave plate 31. Operating in conjunction withthe variable optical element 28 and the wave plate 31, the polarizationanalyzer 32 alters the transmitted amplitude of the polarization-alteredoptical beam as a function of wavelength. This produces an outputoptical beam with transmitted amplitude adjusted as a function ofwavelength.

As will be more fully appreciated in connection with the followingdiscussion, the apparatus of FIG. 4a alters the transmitted amplitude ofthe polarization-altered optical beam in a substantially linear manneron a logarithmic scale. Further, the transmitted amplitude of thepolarization-altered optical beam can be altered in accordance with aselected profile. For example, the device of FIG. 4a can be configuredto dynamically alter the polarization state of the input optical beam tosmoothly and continuously alter the slope profile of the input opticalbeam. In particular, the slope profile can be altered between differentstates selected from a positive slope profile state, a substantiallyflat profile state, and a negative slope profile state.

FIG. 5a shows a Poincare sphere plot of the polarization as a functionof wavelength for light exiting the fixed wave plate. The wavelengthranges from 1530 nm, shown as the large point (33, for example), to 1565nm. The four curves on the sphere correspond to different liquid crystalretardation values (i.e. different voltages applied to the cell). Bydesign, the static wave plate retardation causes the 1530 nm point ofall curves to lie on the meridian passing through the equatorial points34 and 35. This is done by choosing the retardation of the static waveplate to be a whole number of waves at 1530 nm.

When a linear polarization analyzer (e.g., a polarizer) 32 is placedafter the fixed wave plate 31 and oriented with its transmitting axisparallel to the liquid crystal alignment direction 29, the spectraltransmission profile (in dB) develops a nearly linear variation withwavelength as shown in FIG. 5b. The slope is negative and decreases withdecreasing liquid crystal retardation whenever

mλ≦(Δn _(eff))d≦(m+¼)λ  (3)

where m is any integer and <. . . > denotes average over the thicknessof the liquid crystal layer. For the curves in FIG. 5b, m=0. The staticwave plate determines the maximum attainable slope at λ/4: the higherthe order, the larger the slope, as indicated by a larger separationbetween wavelengths on the Poincare sphere. For the curves in FIG. 5b,the static wave plate is 8.5 waves thick for a wavelength of 1530 nm.With the analyzer oriented as shown in FIG. 4a, the static wave platemust be a half-integral number of waves at the prescribed wavelength(1530 nm in this case). However, if the analyzer transmitting directionis rotated by 90 degrees, then similar performance is attained when thestatic wave plate is an integral number of waves. This providesadditional freedom for optimizing the optical performance.

It is also possible to achieve positive slopes, instead of negative,with a simple modification of the design of FIG. 4a. The retardation ofthe static wave plate 31 is chosen to be an integral number of waves atthe maximum wavelength of the interval rather than the minimumwavelength. This is illustrated in FIG. 5c for a retardation of eightwaves at 1565 nm.

The order of the elements in FIG. 4a is shown for descriptive purposes,however, it is understood that the order of the at least one wave plateand the at least one variable element in this or in subsequentembodiments described below can be altered with substantially the sameresult. When it is stated that the wave plate receives or operates onthe polarization-altered optical beam it is understood that thisincludes the configuration whereby the polarized optical beam passesthrough a wave plate before it passes through a variable opticalelement.

For the embodiment just described, although the gain tilt is nearlylinear over a wide dynamic range, there is a significant drawback: theminimum insertion loss is >=3 dB. This shortcoming is overcome with theembodiment shown in FIG. 6a. In this case, a second wave plate 36 (WP2)has been added to the design. The optic axis of WP2 makes an angle ofnominally 10 degrees with the optic axis of wave plate 31 (WP1).Additionally, the transmitting axis of the polarizer 37 is rotated 90degrees about its normal from its orientation in FIG. 4a. With thisconfiguration, the insertion loss can be reduced to less than 1 dB overthe same wavelength range as shown in FIG. 7a. The retardation valuesfor the fixed wave plates WP1 and WP2 were set at 4.5 waves at 1530 nmand 8.5 waves at 1550 nm, respectively. Representativepolarization-versus-wavelength curves are plotted on the Poincare spherein FIG. 7b for different values of the liquid crystal retardation. Togenerate these curves, the liquid crystal retardation was varied between1 and 1.2 waves (at 1550 nm). Unlike FIG. 5a, the curves are notparallel to the polarizer orientation (i.e. the 0-180 meridian line onthe plot). If the liquid crystal retardation is decreased below 1.0wave, polarization-versus-wavelength curves with positive slope areobtained.

FIG. 5c shows curves obtained when the liquid crystal retardation variesbetween 0.7 and 1.0 waves. These curves, in addition to positive slope,have negative curvature. As was observed earlier, this corresponds tothe behavior of the typical EDFA with a static GFF when the gain isincreased, so some of the non-linearity in the gain tilt can becorrected when the gain tilt is negative.

FIG. 8 illustrates the measured results obtained with the embodiment ofFIG. 6a. FIG. 8 confirms the variation in slope and curvature. Thevariable wave plate for the data in FIG. 8 was a 25 micron thick liquidcrystal with the RMS value of the applied AC voltage varied from 2.6 to3.6 volts to achieve a smoothly varying slope from about an average of−0.28 dB/nm to about an average of +0.28 dB/nm over the wavelength rangeof 1528 to 1563 nm. The curvature generally fits the EDFA output byhaving a negative curvature for the positive sloping curves and apositive curvature at higher wavelengths for the negatively slopingcurves.

Additional control over the curvature is achievable with alternativeembodiments of the invention. For example, additional wave plates andadditional variable wave plates at different orientations can be used.As an example, FIG. 9 shows simulated results using three fixed waveplates with one variable wave plate. The wave plates include: one with13.29 waves of retardation at 1550 nm oriented at 45 degrees, one withretardation of 6.62 waves at 1550 nm and orientation of −7 degrees, andone with retardation of 25.97 waves at 1550 nm oriented at 30 degrees.The variable wave plate is a 5 micron thick liquid crystal with theoptic axis at 30 degrees. The analyzer is oriented at 9 degrees. A rangeof profiles is demonstrated with these curves, including a distinctchange in slope near 1540 nm for both the positive and negative slopingcurves. This control of the curvature of the spectral transmittanceprovides an advantage in compensating the spectral profile of an opticalamplifier, such as an EDFA.

Nearly identical performance is achieved for any of the embodimentsdescribed above if the incident linear polarization and the analyzertransmitting direction are simultaneously rotated by 90 degrees. This isthe basis for a polarization independent gain tilt controllerillustrated in FIG. 10a.

FIG. 10a shows an embodiment of the current invention for application ina fiber optic network. This embodiment corresponds to the deviceconfiguration of FIG. 6a. The light from a fiber 38 enters the devicethrough a collimating lens 39. It then passes through a birefringentcrystal 40 that displaces the ordinary 41 (R1) and extraordinary 42 (R2)components of the light by a small distance at the exit to the crystal.The separation depends on the crystal length, birefringence and opticaxis orientation with respect to the propagation direction. Thedisplacement should be sufficient to produce minimal overlap between thetwo beam spots. After exiting the beam displacer, the light passesnormally through a half wave plate 43 (W1). W1 is required to rotate thelinear polarization of both rays by 45 degrees. The two rays, R1 and R2,then pass through the variable wave plate 44 (LC) and two fixed waveplates 45 (W1) and 46 (W2) that serve the same functions as WP1 and WP2described above in conjunction with the embodiment of FIG. 6a. The lightthen passes through a second polarizer (e.g., a birefringent crystal) 47(C2) that is identical to C1. The crystal splits rays R1 and R2 intotheir ordinary and extraordinary components. The extraordinary componentof R1 48 and the ordinary component of R2 49 coalesce at the exitsurface of C2. The recombined beam is focused by a collimating lens 50into the output fiber 51. The other two components 52 and 53 of R1 andR2 are not collected by the optics and are lost at the output.

The two collected rays have nominally the same optical path length sothat in addition to being polarization independent, the device also isfree of polarization mode dispersion, an important consideration forhigh data rate optical transmission.

FIG. 11 shows an embodiment of the current invention wherein the DGTC isincorporated into a fiber optic amplifier circuit to achieve a flattenedgain profile at the output after amplification of a flat input signal54. The nonlinear signal 55 exiting the amplifier 56 is passed through astatic gain flattening filter (GFF) 57. The spectral profile at theoutput of the GFF 58 is then corrected by passage through the gain tiltcontroller 59 to achieve a nearly flat spectral profile 60 for the gainat the output.

Unlike a general gain equalizer, the apparatus of the invention does notprovide arbitrary gain profile adjustment. Instead, the inventionprovides a technique for dynamically controlling the optical gainprofile for a monotonically varying signal. In contrast to a gainequalizer, the apparatus of the invention is inexpensive, small, haslower insertion loss and power consumption, and is simpler to controlbecause there is only one control variable and very little feedbackmonitoring.

The invention is particularly useful in processing optical signals withwavelengths between approximately 1525 and 1565 nm, sometimes referredto as the C-band. The invention is also successfully used in connectionwith the S-band (wavelengths between approximately 1485 and 1520 nm) andthe L-band (wavelengths between approximately 1570 and 1615).

The foregoing description, for purposes of explanation, used specificnomenclature to provide a thorough understanding of the invention.However, it will be apparent to one skilled in the art that specificdetails are not required in order to practice the invention. Thus, theforegoing descriptions of specific embodiments of the invention arepresented for purposes of illustration and description. They are notintended to be exhaustive or to limit the invention to the precise formsdisclosed; obviously, many modifications and variations are possible inview of the above teachings. The embodiments were chosen and describedin order to best explain the principles of the invention and itspractical applications, they thereby enable others skilled in the art tobest utilize the invention and various embodiments with variousmodifications as are suited to the particular use contemplated. It isintended that the following claims and their equivalents define thescope of the invention.

What is claimed is:
 1. An apparatus for processing an optical beam,comprising: at least one variable optical element to dynamically alterthe polarization state of a polarized optical beam to form apolarization-altered optical beam, wherein said polarization-alteredoptical beam includes elliptical polarization; at least one wave plateto process said polarized optical beam, each wave plate having aselected retardation, order of retardation, and orientation; and apolarization analyzer operative in conjunction with said at least onevariable optical element and said at least one wave plate to alter thetransmitted amplitude of said polarization-altered optical beam as afunction of wavelength in accordance with a selected profile, andthereby produce an output optical beam with transmitted amplitudeadjusted as a function of wavelength.
 2. The apparatus of claim 1configured to alter said transmitted amplitude of saidpolarization-altered optical beam in a substantially linear manner on alogarithmic scale.
 3. The apparatus of claim 1 wherein said at least onewave plate is positioned before said at least one variable opticalelement.
 4. The apparatus of claim 1 wherein said variable opticalelement dynamically alters the polarization state of said polarizedoptical beam so as to smoothly and continuously alter the slope profileof said polarized optical beam between different states selected from apositive slope profile state, a substantially flat profile state, and anegative slope profile state.
 5. The apparatus of claim 1 wherein saidvariable optical element is a liquid crystal.
 6. The apparatus of claim1 wherein said variable optical element is an electro-optic birefringentelement.
 7. The apparatus of claim 1 wherein said variable opticalelement is an acousto-optic variable element.
 8. The apparatus of claim1 wherein said variable optical element is a birefringent crystal wedge.9. The apparatus of claim 1 wherein said wave plate has multiple orders.10. The apparatus of claim 1 further comprising a set of wave plates ofpredetermined orders and orientations.
 11. The apparatus of claim 1wherein said wave plate is at an orientation of between approximately 35and 55 degrees with respect to said polarized optical beam.
 12. Theapparatus of claim 1 wherein at least one wave plate has an order ofretardation greater than one.
 13. The apparatus of claim 1 wherein saidpolarization analyzer is a birefringent crystal.
 14. The apparatus ofclaim 1 configured to process a polarized optical beam with wavelengthsbetween approximately 1525 and 1565 nm.
 15. The apparatus of claim 1configured to process an optical beam with a wavelength of approximately1540 nm or less.
 16. The apparatus of claim 1 configured to process anoptical beam with wavelengths between approximately 1485 and 1520 nm.17. The apparatus of claim 1 configured to process an optical beam withwavelengths between approximately 1570 and 1615 nm.
 18. The apparatus ofclaim 1 configured to process an optical beam with amplitude varyingmonotonically with wavelength.
 19. The apparatus of claim 1 furthercomprising a polarizer to process an optical beam to produce saidpolarized optical beam, said polarized optical beam includingorthogonally polarized beams.
 20. The apparatus of claim 19 wherein saidpolarizer is a birefringent crystal.
 21. The apparatus of claim 19wherein said polarization analyzer combines said orthogonally polarizedbeams to produce a recombined beam with an amplitude substantiallyindependent of the polarization of said optical beam.
 22. The apparatusof claim 19 wherein said polarization analyzer combines saidorthogonally polarized beams to produce a recombined beam with adispersion substantially independent of the polarization of said opticalbeam.
 23. The apparatus of claim 1 in combination with a fiber optictransmission line.
 24. The apparatus of claim 1 in combination with anoptical amplifier.
 25. A method of processing an optical beam,comprising: dynamically altering the polarization state of a polarizedoptical beam to form a polarization-altered optical beam, wherein saidpolarization-altered optical beam includes elliptical polarization; andaltering the transmitted amplitude of said polarization-altered opticalbeam as a function of wavelength in accordance with a selected profile,thereby producing an output optical beam with transmitted amplitudeadjusted as a function of wavelength.
 26. The method of claim 25 furthercomprising modifying said transmitted amplitude of saidpolarization-altered optical beam in a substantially linear manner on alogarithmic scale.
 27. The method of claim 25 further comprisingadjusting said polarization state of said polarized optical beam so asto smoothly and continuously alter the slope profile of said polarizedoptical beam between different states selected from a positive slopeprofile state, a substantially flat profile state, and a negative slopeprofile state.
 28. The method of claim 25 utilized to process an opticalbeam with wavelengths between approximately 1525 and 1565 nm.
 29. Themethod of claim 25, utilized to process an optical beam with wavelengthsbetween approximately 1485 and 1520 nm.
 30. The method of claim 25utilized to process an optical beam with wavelengths betweenapproximately 1570 and 1615 nm.
 31. The method of claim 25 utilized toprocess an optical beam with a wavelength of approximately 1540 nm orless.
 32. The method of claim 25 to process an optical beam withamplitude varying monotonically with wavelength.
 33. The method of claim25 further comprising an initial operation of separating an inputoptical beam into orthogonally polarized beams.
 34. The method of claim33 wherein altering includes producing a recombined beam with anamplitude substantially independent of the polarization of said inputoptical beam.
 35. A method of processing an optical beam, comprising:dynamically altering the polarization state of a polarized optical beamto form a polarization-altered optical beam, wherein saidpolarization-altered optical beam includes elliptical polarization; andaltering the transmitted amplitude of said polarization-altered opticalbeam as a function of wavelength, thereby producing an output opticalbeam with transmitted amplitude adjusted as a function of wavelength,wherein said optical beam is processed with amplitude varyingsubstantially monotonically with wavelength.
 36. The method of claim 35further comprising modifying said transmitted amplitude of saidpolarization-altered optical beam in a substantially linear manner on alogarithmic scale.
 37. The method of claim 35 further comprisingmodifying said transmitted amplitude of said polarization-alteredoptical beam in accordance with a selected profile.
 38. The method ofclaim 35 further comprising adjusting said polarization state of saidpolarized optical beam so as to smoothly and continuously alter theslope profile of said polarized optical beam between different statesselected from a positive slope profile state, a substantially flatprofile state, and a negative slope profile state.
 39. The method ofclaim 35 further comprising an initial operation of separating an inputoptical beam into orthogonally polarized beams.
 40. The method of claim39 wherein altering includes producing a recombined beam with anamplitude substantially independent of the polarization of said inputoptical beam.